114 



L. M. Parsons- 



parts of the district show that the dolomitization has been most 

 intense in the immediate neighbourhood of the Harborough Rocks, 

 and that outwards from this centre there is a general fall in the 

 proportion of magnesium southwards towards Carsington and 

 Hopton and westwards towards Roystone. 



The dolomitic gradient is, on the whole, moderate, and only at one 

 part of the area, near Longclifie, have I found a sudden drop in the 

 proportion of magnesium similar to that which occurs in the material 

 near the Wyn Tor, described previously. 



In the district as a whole, the chemical variation is not marked in 

 a vertical direction, the lower beds having much the same com- 

 position as higher strata at the same locality. The metasomatism 

 ends abruptly at the junction of the '" basal dolomite " and the 

 underlying limestones, analyses of the latter showing no magnesium 

 except in the case of the material under the basal dolomite of the 

 Rainster Rocks, which yielded less than 8 per cent of magnesium. 



The figures obtained by dividing the percentage of magnesium 

 carbonate by the porosity do not give a constant, and indicate 

 that prior to leaching the dolomitization was not uniform. 



TABLE ILLUSTRATING VARIATION IN CHEMICAL COMPOSITION, SPECIFIC 

 GRAVITY, AND POROSITY OF DOLOMITES BETWEEN PARWICH MOOR AND 



WIRKSWORTH.i 



Field relations, certain inherent structures, and chemical com- 

 position suppty a large balance of evidence indicating that the 

 dolomitization between Parwich and Wirksworth is of a subsequent 

 kind produced by the influence of ground waters. The fact that in 

 the whole of Central Derbyshire the maximum magnesiation is 

 towards the east gives considerable support to the theory of Permian 

 influence. While the magnesian material of the Winster, Matlock, 

 and Harborough districts may have had a common source of origin, 

 there is no proof that the metasomatism in any one of these districts 

 was simultaneous with that in any other. 



^ For considerable help in the chemical part of the work I am indebted 

 to Messrs. A. G. Brighton, C. Jenkins, and H. D. Thomas. 



